Because of smoke and "pop, pop, pop" noises coming from the Nissan Pathfinder parked in the heart of Times Square, alert street vendors knew to flag down a police officer, averting catastrophe.

While the celebration that has ensued is understandable, the incident this past weekend is actually a sobering reminder of just how vulnerable we are.

The combination of aggressive law enforcement and plain luck have prevented any major, successful attack in the United States since Sept. 11, 2001, but we cannot expect our good fortune simply to continue indefinitely.

Even though the Pakistani Taliban has claimed credit for the car bomb, there is not substantial enough evidence to confirm their boast. It should be of greater concern, however, if the suspected bomber, Faisal Shahzad, was a “lone wolf” with little or no outside support or training, given how close the car bomb came to wreaking havoc on perhaps the most instantly recognizable neighborhood in America, if not the world.

Consider that Times Square is a true hard target, with highly trained police officers on every corner. Not only that, but it’s in the center of a city protected by the New York Police Department’s counterterrorism unit, which is easily the best in a local police force, and arguably even more effective than the FBI’s.

Yet, had the bomb been made correctly and detonated as presumably intended, a “significant fireball” could have claimed dozens or even hundreds of lives on the busiest night of the week in Times Square.

As heroic as the street vendors and the responding police officers were, luck was still the single biggest factor in averting disaster.

Luck has been essential in several close calls. The Christmas Day underwear bomber, Umar Farouk Abdulmutallab, was able to get his explosives past security, but it was our luck that he could not ignite his bomb. Similarly, shoe-bomber Richard Reid evaded security, only to fail in detonating his explosives on the flight.

In October 2005, University of Oklahoma student and Islamic convert Joel Hinrichs III detonated himself less than 200 yards from the football stadium during a Sooners football game. Over 80,000 people were inside. It stands to reason that Hinrichs, who reportedly attended the same mosque as the would-be 20th hijacker, Zacarias Moussaoui, had something much grander in mind than mere suicide.

Since Sept. 11, there have been more than 800 terror-related arrests in the United States, according to New York University's Center on Law and Security. The onslaught is constant.

Two key factors have enabled law enforcement to protect us:

• First is that most of the plots thus far have involved either existing networks or people reaching out to terrorist organizations or known radical communities, giving the FBI a chance to monitor or infiltrate the terrorist plots.

• Second is a cultural shift away from the mentality that prevented the FBI from seeking a search warrant to inspect Moussaoui's laptop, which the agency had in its possession nearly a month before September 11.

Unfortunately, both of these ingredients are endangered.

Radical messages of Islamic victimization at the hands of an evil America (or an evil Israel, with the help of America) abound on the Internet and in Muslim communities across the U.S.

The sense that the Islamic world is under attack by the West has been the stated motivation of most captured jihadists, who believe they are acting quite nobly, as they see it, in defending supposedly defenseless fellow Muslims.

Law enforcement's success in preventing attacks—and consequently, jihadists’ failure in executing them—only increases the odds of a “lone wolf” deciding to take matters into his own hands in defending Muslims. Lone wolves are naturally harder to track, as they can more easily stay off the radar until they act.

Despite the preposterous findings of a Department of Defense report this January, an abundance of evidence suggests that Maj. Nidal Malik Hasan went on a shooting spree last year at Fort Hood in order to, quoting from his own Internet posting about the virtues of suicide bombers, “help save Muslims by killing enemy soldiers.”

Based on what the public learned about the government’s knowledge of Maj. Hasan’s words and deeds before his murderous rampage, it is clear he never should have been in a position to wage his jihad in the first place. Debilitating political correctness kept him in uniform, as his superiors were more worried about bad PR or potential lawsuits than dealing with an obvious threat.

That’s why leadership matters. President Obama, to his credit, has not enacted many changes to the Bush administration counterterrorism policies. But the rhetorical shift has been pronounced.

Given the Muslim background of his father and stepfather, and his own formative years in predominantly Islamic Indonesia, President Obama actually has the best position of any U.S. leader to tackle Islamic radicalism head-on. So far, however, he has declined to do so. His administration has instead engaged in a coordinated campaign to soft-pedal the threat of radical Islamic ideology.

Most likely, the number of potential lone wolves will grow over time. As we could have learned in far more painful fashion this weekend, even the best policing can fall short. The key to stopping lone wolves is defeating the ideology that motivates them to act.